1. Field of the Invention
The present invention relates to a power-controlling semiconductor integrated circuit which includes a current detection circuit and controls a power depending on detected current, and relates to, for example, an effective technique when being used in a charge-controlling IC (semiconductor integrated circuit) mounting a charge-controlling circuit.
2. Description of Related Art
A charging apparatus for a secondary battery uses an IC having a charge-controlling circuit to control a charging current with a charge-controlling transistor composed of a MOSFET (insulated gate field effect transistor; hereinafter referred to as MOS transistor) provided between an input terminal to which a direct voltage from an AD adapter is input and an output terminal to which the secondary battery is connected.
Conventionally, in such charge-controlling IC, current flowing through a current-controlling transistor is detected during a preliminary charge or rapid charge so that charging current is controlled to be constant. As a method for detecting charging current in such constant current mode, there has been known a method where a current-detecting sense resistance is connected in series with a current-controlling transistor so that current is detected based on a voltage drop amount in the resistance This method can detect current with a relatively-high degree of accuracy, but since current flowing through the sense resistance is large, there is a problem that a power loss in the sense resistance is large so that power efficiency lowers.
Consequently, there has been proposed a method where a transistor of smaller size than a current-controlling transistor is connected in parallel with the current-controlling transistor, a gate voltage same as that of the current-controlling transistor is applied to the smaller transistor to generate current which is scaled down to charging current in a current mirror to flow the generated current through the sense resistance so that current is detected based on a voltage drop amount in the resistance This method has an advantage that power efficiency is improved because the current flowing through the sense resistance is small, but there is a problem that since a bias condition of the current-detecting transistor is different from that of the current-controlling transistor due to load variation or the like, it is impossible to flow accurately scaled-down current, and a detection accuracy lowers.
On the other hand, there has been proposed an invention to improve current detection accuracy in driving/controlling of a power transistor by providing a current-detecting transistor current-mirror-connected to the power transistor, and a bias-controlling transistor connected in series with the current-detecting transistor, by providing a difference amplifier whose inputs are drain voltages of the power transistor and the current-detecting transistor, and whose output terminal is connected to a gate terminal of the bias-controlling transistor, so that a bias condition of the current-detecting transistor becomes same as that of an outputting transistor by an imaginary short function of the difference amplifier (Japanese Patent Application Laid-Open Publication No. 2004-259902).
The inventors of the present application have been tried to apply a current detection technique of current mirror system using an operation amplifier to the charge-controlling circuit. As a result, it becomes clear that though current detection with a relatively-high degree of accuracy can be performed when one of a plurality of cells is used for current detection in driving/controlling of a multi-cell type power MOSFET, there is a problem that in the case of a charge-controlling semiconductor integrated that the inventers intend to apply, where the current-controlling transistor and the control circuit thereof are formed on one semiconductor chip, variation in manufacturing processes misaligns a size ratio of the current-controlling transistor and the current-detecting transistor and a desired value so that current detection accuracy lowers.